In a screening machine a drive imparts a screening motion to a screen deck to separate, sift, or classify particles of different sizes, weights, and/or shapes. Typically the drive is mounted to a base and has a moving element, armature, or rotor which is connected to the screen deck to shake, oscillate or gyrate the deck relative to the base.
Some types of screeners have a linear drive, for example an electromagnet, whereby the screen deck is vibrated back and forth in an essentially straight line screening motion.
This invention, however, concerns screeners of the so-called "gyratory" type. In most gyratory screeners the screening motion has different amplitudes at different points on the screen deck, along two perpendicular axes. The motion may for example be circular at one end of the deck but nearly linear at the other end. The deck may be driven by a rotating crank pin at an upper, head, or feed end while the lower, tail, or discharge end is constrained to move in a nearly straight line path. The intermediate part of the deck, near its center of gravity, moves in an elliptical path. Usually but not necessarily the elliptical path of motion of a gyratory screener, measured near the center of gravity of the deck, has an amplitude which is substantially greater in the longitudinal direction than in the lateral (crosswise) direction.
Gyratory screeners are widely used because gyratory motions are considered to offer a distinct advantage in screening, in comparison to either a reciprocating motion or a purely circular motion. The particles are more effectively stratified, rolled over one another and shifted about, which improves the screening efficiency. Moreover, the incoming particles are more uniformly distributed over the screen at the feed end, and the removal of near-size particles at the discharge end is markedly improved.
One well known type of screener having a gyratory motion is sold under the "Rotex" trademark. In "Rotex" gyratory screeners the drive (which is mounted to the base) rotates a crank pin which is journaled in the head end of the screen deck. Rotation of the crank pin by a drive motor imparts a circular motion to the head end. At the discharge end a swing link or "drag arm" is connected to the deck to constrain its movement to a more or less reciprocating or linear motion. The middle portion of the deck moves in an elliptical path in which the component of movement along the longitudinal axis or direction of the deck is substantially greater (for example, about two times greater) than that in the transverse direction.
The drive of a gyratory screener is connected between the base of the machine and the deck, and the force exerted by the drive on the deck creates an equal and opposite reaction force on the base, which tends to oscillate the base oppositely from the deck. If the base is rigidly mounted to a fixed support structure (for example, if the base is bolted to the floor of a building) this oscillating reaction force on the base is imparted directly to the support or building itself, and can set up a powerful vibration in the building. The vibration of the base of a large screener can impart an undesirably large and possibly dangerous vibration to the building housing the screener.
In order to reduce the affect of the base reaction force on the building or other machine support, various means have been used to isolate the base from the support structure. Motor driven counterbalances have been added to linear screeners, see for example Overstrom U.S. Pat. No. 2,358,876. Alternatively, the base might be resiliently supported on shear mounts (such as rubber blocks), or suspended on cables. (Machines having a suspended or shear mounted base are referred to herein as "moving base" machines because the base is not fixed rigidly to a support but rather can move relative to the support.) Such mounts permit the base to move in response to the reaction forces imparted to it by operation of the drive.
If shear mounts are to be used, in order to effectively isolate the motion of a screener from its support structure, the shear mounts should have a natural frequency no more than about 1/3 of the e screener's operating frequency. However, shear mounts are generally so stiff that they do not have a natural frequency within that desired range. (If suitably "soft" shear mounts were chosen to isolate the screener, the resulting system would be statically unacceptable.) Thus, in practice shear mounts do not isolate the screener but rather transmit the unbalanced forces to the underlying support structure. For these reasons shear mounts are typically an ineffective means of attempting to isolate a screener from its support structure.
In contrast, a cable suspension can effectively isolate a screener from its support; and many if not most large capacity gyratory screeners are cable suspended in order to prevent the undesirably powerful base vibrations from being transmitted to the structure housing the screener.
Although mounting the screener base for movement relative to its support can effectively isolate the support from the vibration, as explained above, such movable mounting of the base has an adverse affect on the motion of the screen deck: the relative base movement offsets and reduces the movement of the deck relative to ground or other fixed support structure. As the drive moves the screen in one direction, the reaction force imparted by the drive to the base tends to move the base in the opposite direction, which reduces the net motion of the screen relative to the ground (the "screen-to-ground" relative motion). However, it is the screen-to-ground relative motion which effects particle separation; therefore, base movements which offset screen-to-ground movement reduce the screening efficiency of the machine. In short, the base movement of a moving-base screener (including both cable-hung and shear-mounted screeners) offsets the screening movement and thereby reduces screen efficiency and machine capacity.
Various means have been used to reduce the reaction force of a gyratory drive on the base. So-called "single counterbalance" drives, in which an opposed counterbalance weight rotates with the crank, are used for this purpose. However, a single counterbalance does not eliminate all the reaction force on the base because in a gyratory screener the drive force and the reaction force have different amplitudes along different axes, and a single counterbalance cannot offset the differing motions along both axes. For example, if a single counterbalance is sized to eliminate the longitudinal reaction force acting on the base (usually the larger of the two force components), it will overcompensate for the lateral reaction force and will thereby set up an unbalanced lateral force that itself acts on the base. Relatively small single counterbalance screeners, that is, those having a "swung weight" (the weight of that part of the machine that moves relative to ground) of less than about 800 pounds, can be mounted directly to a "fixed" support without imparting undue vibration to the support structure. However, for gyratory screeners having greater swung weights, the screener is usually cable suspended or otherwise isolated from the support structure in order to isolate the unbalanced force. As already explained, however, when this is done the resulting motion of the base causes an undesirable reduction in screener efficiency.
So called "double counterbalance" drives are also known. Simpson U.S. Pat. No. 1,668,984 teaches a gyratory screener having two counter-rotating counterbalance weights operated by the drive. Because of the counter-rotation, twice every revolution the weights move through the same angular position, at which their generated forces are additive; and twice every revolution they pass through positions that are diametrically opposite, at which their generated forces are subtractive. If the counterbalances are positioned so that their forces add along the longitudinal axis and subtract along the lateral axis, they can be sized so that the additive force is substantially equal to the longitudinal out-of-balance force and the difference between their forces is substantially equal to the lateral out-of-balance force.
In a relatively small screener, a double counterbalance drive can reduce base vibration sufficiently that the base can be safely bolted directly to the floor. However, even with a double counterbalance a large screener is usually cable hung in order to isolate the base movement from a building structure. Even a double counterbalance drive cannot neutralize the base reaction forces in a gyratory screener as effectively as is desired. The gyratory motion has some force components that are not fully offset, especially at the lower end of the deck. As a result, the base of a suspended conventional screener still has an undesirable vibration relative to a fixed surface. By way of example, the drive crank of a Rotex Series 70 screener moves the deck, adjacent the pin journal, in a circle of about 3.5" diameter. Even with a double counterbalance, the base of a cable hung screener, measured at the head end, moves in a loop path having x-y dimensions of about 0.31-0.38". Since this base motion is 180.degree. out-of-phase with the deck motion, it reduces the screen motion from about 3.5" to as little as about 3.12", a loss of almost 11% of the stroke. Even though cable mounting the base prevents this base movement from being transmitted to the building, screening efficiency is nevertheless significantly reduced. (Increasing the amplitude of the movement imparted to the screen deck by the drive would improve the screening motion, but is not practical because it would be more costly and would increase the out-of-balance forces acting on the base.)
On a given machine a double counterbalance does not entirely eliminate the motion of a movable base, but it reduces deck movement less than a single counterbalance would. It is thus desirable to use a double counterbalance drive on larger moving use a double counterbalance drive on larger moving base machines. However, the cost of a double counterbalance drive is substantially greater than that of a single counterbalance. Furthermore, double counterbalancing requires an additional set of gears and bearings, adds complexity, and requires additional lubrication and maintenance.
Thus, a substantial need has existed to minimize the base movement of both single and double counterbalanced gyratory screeners of the movable base type, in order to increase the relative movement of the deck and thereby improve the efficiency.